1. Field of the Invention
The present invention relates to an enzyme electrode, and more particularly relates to an enzyme electrode having an enzyme immobilized on a conductive member through a π-conjugated metal complex molecule and an application field thereof.
2. Description of the Related Art
An enzyme, which is a protein biocatalyst made in a living cell, exhibits a stronger activity under milder conditions than does a normal catalyst. The enzyme also exhibits substrate specificity, and each enzyme generally catalyzes only a specific reaction of a specific substrate. Among enzymes, an enzyme referred to as an oxidoreductase catalyzes the oxidation-reduction reaction of a substrate. If an electric charge produced through the reaction due to the oxidoreductase could be provided to a conductive member, a highly selective electrode with a low overpotential can be produced by making use of this feature of the enzyme.
However, most oxidoreductases have a form in which the redox center is confined deep inside a three-dimensional structure of a protein. For this reason, the redox center is too far from the conductive member to effectively transfer electrons between it and the conductive member. As a result, it has been generally difficult to directly transport electrons between the active site of the oxidoreductase and the conductive member.
In order to enable the direct electron transfer, a technique has been widely used in which the enzyme is electrically connected to the conductive member with a material referred to as a mediator. For instance, Adam Heller J. Phys. Chem. 1992, 96, 3579 to 3587 describes a technique of delivering a mediator molecule into a protein of the enzyme and making the mediator molecule transfer electrons between itself and the redox center of the enzyme only after the mediator molecule is in close proximity to the redox center of the enzyme. After having transferred electrons between the mediator and the active site of the enzyme, namely, the redox center, the mediator transports the electric charge to the conductive member through diffusion and electron hopping. Thus, the electric charge of the enzyme reaction is delivered to the conductive member.
For the purpose of effectively using an enzyme and a mediator, and continuously using an electrode for a long period of time by preventing these components from leaking into the system, a technique of immobilizing them onto the electrode has been proposed. U.S. Pat. No. 5,262,035 discloses a technique of chemically or electrostatically immobilizing the enzyme and the mediator by using an enzyme carrier having the mediator in the molecule.
This technique shows a higher retaining ability of the electrode for the enzyme and the mediator and a greater effect of preventing them from leaking out of the system than a technique of physically adsorbing the enzyme.
Aside from these techniques, an enzyme electrode has been reported which uses an electroconductive polymer to immobilize the enzyme onto an electrode.
Japanese Patent Application Laid-Open No. 562-115284 discloses a technique of entrapping and immobilizing the enzyme in a polypyrrole film of an electroconductive polymer by electrolytically polymerizing pyrrole in a solution containing pyrrole and an oxidoreductase. According to this technique, an enzyme electrode can be prepared by immobilizing the enzyme in the polypyrrole of the electroconductive polymer without particularly needing a mediator.